document span_matcher.h

Author: dgoffredo

src/datadog/span_matcher.h

@@ -1,5 +1,14 @@
 #pragma once
 
+// This component provides a `struct`, `SpanMatcher`, that is used as a base
+// class for `TraceSamplerConfig::Rule`, `SpanSamplerConfig::Rule`, and related
+// types. A `SpanMatcher` is a pattern that a given span either matches or
+// doesn't, depending on that span's service, operation name, resource name, and
+// tags. The member function `SpanMatcher::match` returns whether a specified
+// span matches the pattern.
+//
+// `SpanMatcher` is composed of glob patterns. See `glob.h`.
+
 #include <string>
 #include <unordered_map>
 
@@ -15,6 +24,8 @@ struct SpanMatcher {
   std::string service = "*";
   std::string name = "*";
   std::string resource = "*";
+  // For each (key, value), the tag's key must be present and match literally
+  // (no globbing), while the tag's value must match the glob pattern.
   std::unordered_map<std::string, std::string> tags;
 
   bool match(const SpanData&) const;